This invention relates to manufacturing coated optical fibers and in particular to a novel process for applying multiple coatings to the bare optical fibers.
Optical fibers typically are silica-based. To improve the moisture resistance and mechanical properties of the fiber, the fibers are often coated with multiple polymeric coatings disposed concentrically about the fiber, with the coating nearest the fiber generally being more flexible than the outermost coating(s).
To form the coatings, a photopolymerizable composition typically is applied to the fiber and polymerized by exposure to actinic radiation, e.g., ultraviolet radiation, to form a first polymer coating. Next, a second photopolymerizable composition is applied to the first polymer coating and likewise exposed to actinic radiation to form a second polymer coating.
Optical fibers are used in a variety of environmental conditions, including a wide range of temperatures. Typical operating temperature ranges may vary from 80xc2x0 C. to xe2x88x9250xc2x0 C. The possible rate temperature change of many of these optical fibers may be several tens of degrees per minute. Furthermore, optical fibers are many times placed inside of complex devices, structures, underneath roadways, in submarine applications, or in other locations where access and repair presents great challenges and costs. Also, safety devices, such as communications, guidance systems (e.g., gyroscopes), and sensors, depend on the accurate functioning of these fibers.
One problem that has been observed with such coated fibers is that under certain environmental conditions and/or tensile stresses, the first polymer coating fractures, or delaminates, or both, thereby compromising the strength and moisture resistance of the fiber. In addition, in the case of telecommunications fibers and stress-sensitive fibers such as polarization maintaining (PM) and polarizing (PZ) fibers, the fibers manifest microbending losses or other effects on the optical signal that degrade the overall performance of the fiber. In addition to the lack of system reliability caused by these fiber failures, since repair or replacement of these fibers is often extremely difficult and costly, the failures may threaten entire communication networks.
By first discovering the reasons for these failures, the present invention then discloses a novel method for manufacturing multi-coating optical fibers having a larger temperature operating range.
In a first aspect, the invention features a method for coating an optical fiber that includes: (a) applying a photopolymerizable composition to an optical fiber having a surface coated with a first polymer coating; and (b) exposing the photopolymerizable composition to a source of actinic radiation to form a second polymer coating under conditions which inhibit the production of thermally induced tensile stresses in the first polymer coating. The term optical fiber is meant to include both bare silica-based and polymeric fiber waveguides as well as coated or partially coated bare fibers.
In preferred embodiments, the fiber is cooled prior to application of the photopolymerizable composition. Preferably, this is accomplished by exposing the fiber to a chilled stream of gas (e.g., an inert gas such as helium).
Inhibiting the production of thermally induced tensile stresses in the first polymer coating during exposure may be accomplished in several ways. For example, the fiber may be cooled with a chilled stream of gas such as helium during exposure. Another protocol involves providing the source of actinic radiation with a dichroic reflector that transmits infrared radiation generated by the radiation source away from the fiber. Yet another useful protocol includes placing a water-cooled jacket concentrically about the fiber. The surface of the jacket may be further provided with an infrared radiation-absorbing coating. In another embodiment, a tube (e.g., a quartz tube) having a surface coated with an infrared radiation-absorbing coating is disposed concentrically about the fiber.
Each of these protocols may be used alone, or in combination with any, or all, of the others.
The actinic radiation preferably is ultraviolet radiation. The first polymer coating preferably includes an acrylate-functional silicone polymer, while the photopolymerizable composition preferably includes a photopolymerizable acrylate-functional epoxy or acrylate-functional urethane composition.
In a second aspect, the invention features a method for coating an optical fiber featuring a surface coated with a first polymer coating where the fiber is essentially free of a hermetic carbon coating underlying the first polymer coating. The method includes (a) cooling the fiber (e.g., by exposing the fiber to a chilled stream of gas such as helium gas); (b) applying a photopolymerizable composition to the first polymer coating; and (c) exposing the photopolymerizable composition to a source of actinic radiation to form a second polymer coating. Preferably, the method further includes inhibiting the production of thermally induced tensile stresses during exposure according to the procedures described above.
The invention provides optical fibers having multiple polymer coatings in which the production of tensile stresses within an individual polymer coating is minimized. The fibers exhibit good moisture resistance and mechanical properties, and resist delamination. The ability to minimize tensile stresses, and thus the defects associated with such stresses, makes the fibers particularly useful in defect-sensitive applications such as interferometric fiber optic gyroscopes.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.